The colors appear too bright, or vivid. There are many guys doing case hardening heat treatment, and while each have developed their own packing medium, whether it be bone, bone meal, leather or whatever, I think it's a safe guess that they are all using electric heat treat ovens, or possibly one of the good natural gas fired units.
I used to get an order from Colts, about every two years for a few dozen 'packing boxes' cast from a heat resistant alloy (CA6-NM) These boxes were 5x5x10", with cover plates that were wedge-clamped with a flat cover. I went to their heat treat dept several times and know they were using the same oil fired retorts (furnaces) that were pre 1900.
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